Method for computing a downlink beamforming weighting vector based on up link channel information

ABSTRACT

The present invention discloses a method for obtaining a downlink beamforming weighting vector in a wireless communications system based on channel information about an uplink channel. The method comprises obtaining the channel information about the uplink channel by a means selected from the group comprising of training signals, pilot signals, and data signals, wherein the uplink channel comprises a set of uplink sub-channels, calculating a spatial signature of the uplink channel with the channel information, and computing a downlink beamforming weighting vector of a downlink channel with the spatial signature of the uplink channel, wherein the downlink channel comprises a set of downlink sub-channels that share few or no sub-carriers with the set of uplink sub-channels.

CROSS REFERENCE

The present application claims the benefit of U.S. ProvisionalApplication Ser. 60/847,181, which was filed on Sep. 26, 2006.

BACKGROUND

The computation of a downlink (DL) beamforming weighting vector is basedon channel information in the downlink direction. In a time divisionduplex (TDD) system, the channel information in the downlink directionbecomes available as long as the uplink (UL) channel information isknown. This is true due to the reciprocal nature of the DL and UL TDDchannels. However, in a frequency division duplex (FDD) system, thiskind of reciprocal characteristic does not exist between the DL and ULchannels. As a result, the information about the DL channel must be sentback to a base transceiver station (BTS) explicitly by a mobile station(MS).

In orthogonal frequency division multiplexing (OFDM) and orthogonalfrequency division multiple-access (OFDMA) systems, the carriers of OFDMsymbols may experience different levels of impairment. Whenever there isa significant change in the channel quality of a sub-carrier, the MSmust send channel information back to the BTS explicitly.

In a TDD OFDMA system, the frequency separation between the DL and ULchannels might vary from a fraction of a MHz to a few MHz. This is dueto the fact that the BTS scheduler assigns a sub-carrier (frequency) tothe DL and UL channels dynamically.

For example, in the TDD version of IEEE 802.16 d/e (WiMax) standard, theDL and UL channels both operate in one of the following frequency bands,i.e., 2.5 MHz, 5 MHz, 10 MHz and 20 MHz. The DL channel is divided intosub-carries, any number of which could form a sub-channel. A permutationis designed to minimize the probability of reusing the sub-carriers inadjacent cells.

Depending on which permutation is used, the DL and UL channels may havefew or no sub-carriers in common. FIG. 1 is a diagram illustrating anarbitrary assignment of sub-carriers in the UL and DL channels in atwo-dimension diagram of time and frequency domains.

In FIG. 1, a radio channel is divided into 24 sub-carriers 110, each ofwhich is represented by an empty block. Nine of the 24 sub-carriers areassigned to a BTS in a cell for downlink traffic. The nine sub-carriesare grouped into six sub-channels 120, each of which is represented by ablock with dots. Six of the 24 sub-carriers are assigned to an MS foruplink traffic. The six sub-carries are grouped into five sub-channels130, each of which is represented by a block with horizontal lines. Eachof the sub-channels is composed of one or more sub-carriers.

Although the frequency separation between the UL and the DL channels issmall, the BTS cannot use UL channel information to estimate the DLchannel condition with the traditional methods.

As such, what is desired is a method for computing a DL beamformingweighting vector based on UL channel information in a TDD OFDMA systemwhere there is little or no overlap between the sub-carriers in the ULand the DL channels.

SUMMARY

The present invention discloses a method for obtaining a downlinkbeamforming weighting vector in a wireless communications system basedon channel information about an uplink channel. The method comprisesobtaining the channel information about the uplink channel by a meansselected from the group comprising training signals, pilot signals, anddata signals, wherein the uplink channel comprises a set of uplinksub-channels, calculating a spatial signature of the uplink channel withthe channel information, and computing a downlink beamforming weightingvector of a downlink channel with the spatial signature of the uplinkchannel, wherein the downlink channel comprises a set of downlinksub-channels that share few or no sub-carriers with the set of uplinksub-channels.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof, will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

The drawings accompanying and forming part of this specification areincluded to depict certain aspects of the invention. The invention maybe better understood by reference to one or more of these drawings incombination with the description presented herein. It should be notedthat the features illustrated in the drawings are not necessarily drawnto scale.

FIG. 1A is a diagram illustrating an arbitrary assignment ofsub-carriers in the UL and DL channels.

FIG. 2 is a flow diagram illustrating a method for computing a downlinkbeamforming weighting vector by using time-domain channel impulseresponse function.

FIG. 3 illustrates neighborhoods of one or more UL sub-channels.

FIG. 4 is a flow diagram illustrating a method for computing a downlinkbeamforming weighting vector by selective interpolation orextrapolation.

DESCRIPTION

The following detailed description of the invention refers to theaccompanying drawings. The description includes exemplary embodiments,not excluding other embodiments, and changes may be made to theembodiments described without departing from the spirit and scope of theinvention. The following detailed description does not limit theinvention. Instead, the scope of the invention is defined by theappended claims.

The present invention discloses a method for computing a downlink (DL)beamforming weighting vector in a time division duplex (TDD) orthogonalfrequency division multiple-access (OFDMA) system without requiring amobile station (MS) to send DL channel information to a base transceiverstation (BTS) explicitly. The DL beamforming weighting vector iscomputed by using uplink (UL) channel information even when the UL andthe DL channels share few or no sub-carriers. It is known to a personwith skills in the art that in a situation where some sub-carriers areused for both UL and DL traffic, the complex conjugate of the UL channelcoefficient (possibly scaled with a complex number) provides an optimalDL beamforming weighting vector.

In different scenarios, DL beamforming weighting vectors might becomputed using a more complex function than the one described above.Regardless of which function is used, the UL channel coefficients play amajor role.

Assume that one UL channel is divided into S sub-channels {f₁ f₂ . . .f_(S)}, each of which is composed of a number of sub-carriers. ThePartially Used Subchannelization (PUSC) permutation in IEEE 802.16 e/d(WiMax) is one example of a sub-carrier assignment.

A channel impulse response function is defined by the followingequation:

${{h(t)} = {{{a_{1}{\delta \left( {t - \tau_{1}} \right)}} + {a_{2}{\delta \left( {t - \tau_{2}} \right)}} + \ldots + {a_{M}{\delta \left( {t - \tau_{M}} \right)}}} = {\sum\limits_{i = 1}^{M}{a_{i}{\delta \left( {t - \tau_{i}} \right)}}}}},$

where τ_(i) is the delay time of the i-th multi-path component anda_(i), a complex number, is the amplitude of the i-th multi-pathcomponent. The channel impulse response function h(t) includes allmulti-path components with non-zero energy up to the delay time τ_(M).

For example, a channel might have six multi-path components with thelargest delay time equal to 14 times of the sampling rate, i.e.,τ_(M)=14. The channel impulse response function h(t) has six terms, eachof which corresponds to a multi-path component, and the amplitudes a_(i)of the remaining eight terms are set to zero. The delay time of amulti-path component is a multiple of the sampling interval. If thedelay time is not an integer, it is mapped to the next integer that is amultiple of the sampling interval.

FIG. 2 is a flow diagram illustrating a method for computing a DLbeamforming weighting vector in accordance with one embodiment of thepresent invention. This method is used to calculate a DL beamformingweighting vector when the S sub-channels {f₁ f₂ . . . f_(S)} in the ULchannel are spread over the entire frequency band of a radio channel,and the S is large enough, compared with the number of the multi-pathcomponents.

In step 210, the UL channel coefficients are obtained from a covariancemethod or other conventional approaches, using training signals, pilotsignals, or data signals.

In step 220, by using the UL sub-carrier channel coefficients, thecoefficients of the time-domain channel impulse response function h(t)are calculated based on a relationship between the frequency-domainchannel coefficients and the time-domain channel impulse responsefunction h(t). This relationship is represented by the following matrixequation:

${\begin{pmatrix}r_{g_{1}} \\r_{g_{2}} \\\vdots \\r_{g_{W}}\end{pmatrix} = {\begin{pmatrix}1 & {\exp \left( {{- {j2\pi}}\frac{g_{1}}{F}} \right)} & {\exp \left( {{- {j2\pi}}\frac{2g_{1}}{F}} \right)} & \cdots & {\exp \left( {{- {j2\pi}}\frac{\left( {M - 1} \right)g_{1}}{F}} \right)} \\1 & {\exp \left( {{- {j2\pi}}\frac{g_{2}}{F}} \right)} & {\exp \left( {{- {j2\pi}}\frac{2g_{2}}{F}} \right)} & \cdots & {\exp \left( {{- {j2\pi}}\frac{\left( {M - 1} \right)g_{2}}{F}} \right)} \\\vdots & \vdots & \vdots & \; & \vdots \\1 & {\exp \left( {{- {j2\pi}}\frac{g_{W}}{F}} \right)} & {\exp \left( {{- {j2\pi}}\frac{g_{W}}{F}} \right)} & \cdots & {\exp \left( {{- {j2\pi}}\frac{\left( {M - 1} \right)g_{W}}{F}} \right)}\end{pmatrix}\begin{pmatrix}a_{1} \\a_{2} \\\vdots \\\vdots \\a_{M}\end{pmatrix}}},$

where r_(g) _(i) is the receiving signal on frequency g_(i), of asub-carrier and F is the size of the Fast Fourier Transform (FFT) of anOFDMA system.

Depending on the structure and distribution of S disjoint sub-channels{f₁ f₂ . . . f_(S)}, it is advantageous to combine a predeterminedneighboring sub-carriers to form a more reliable set of W disjointsub-channels {g₁ g₂ . . . g_(W)}.

If the S disjoint sub-channels {f₁ f₂ . . . f_(S)} are well dispersed,then a set of W disjoint sub-channels {g₁ g₂ . . . g_(W)} is the same asa set of {f₁ f₂ . . . f_(S)}. In other words, S equals W.

However, if two or more sub-channels f_(i){f₁ f₂ . . . f_(S)} arecomprised of a set of adjacent sub-carriers, it might be beneficial tocompute the average of the receiving signals of the set of adjacentsub-carriers and assign the average signal to one sub-channel denoted byg_(i). By doing so, the number of sub-channels is reduced and W<=S.

The equation described above represents an FFT operation on the channelimpulse response function h(t) of the W disjoint sub-channels {g₁ g₂ . .. g_(W)} in the UL channel. The equation can be solved by using matrixoperations such as the inverse or pseudo-inverse of the matrix shown in0025, or by using estimation techniques such as the maximum likelihood,the minimum mean squares error, or the maximum a posteriori method.

In step 230, after determining the time-domain channel impulse responsefunction h(t) for each of the antennas in the antenna array based on theabove equation, the frequency response of the channel can be obtained bytaking the FFT of h(t). Subsequently, the spatial signature of a channelis obtained and a DL beamforming weighting vector is calculated.

Since the BTS has no prior knowledge about the actual maximum multi-pathdelay, the BTS might assume that the maximum multi-path delay M is equalto W. If the maximum multi-path delay M is larger than W, thetime-domain channel impulse response function h(t), obtained based onthe above equation, may differ from the actual channel impulse response.The difference between the time-domain channel impulse response functionh(t) and the actual channel impulse response depends on the signalstrength of the multi-path components with delay time larger than Mtimes the sampling rate. The beamforming weighting vector is computedaccording to the approximated time-domain channel impulse responsefunction h(t).

FIG. 3 illustrates a neighborhood 340 of a UL channel 330. For asub-channel 330 in a set of S disjoint sub-channels {f₁ f₂ . . . f_(S)}in the UL channel, its neighborhood 340 is composed of a predeterminednumber of sub-carriers f_(N).

The relationship between the UL sub-channel 330 and the DL sub-channel320 is illustrated by dashed lines drawn from the UL sub-channel 330 tothe DL sub-channel 320 in FIG. 3.

If the neighborhood of one UL sub-channel 350 overlaps with that ofanother UL sub-channel 360, the neighborhood could be redefined as anasymmetric neighborhood but it is still based on the center of the ULsub-channel to resolve ambiguity.

FIG. 4 is a flow diagram illustrating a method for computing a DLbeamforming weighting vector by selective interpolation orextrapolation.

In step 410, a BTS identifies the neighborhood of one UL channel, asillustrated in FIG. 3.

In step 420, the DL sub-carriers that fall within any of theneighborhoods of the UL sub-channels are identified. A DL beamformingweighting vector is obtained by using the DL sub-carrier channelinformation.

In step 430, the DL sub-carriers fall outside the neighborhoods of theUL sub-channels. Interpolation or extrapolation techniques (eitherlinear or non-linear, depending on the tradeoff between complexity andperformance) are used to calculate a DL beamforming weighting vectorbased on the channel information, about the immediate neighboring ULsub-channels.

The above illustration provides many different embodiments orembodiments for implementing different features of the invention.Specific embodiments of components and processes are described to helpclarify the invention. These are, of course, merely embodiments and arenot intended to limit the invention from that described in the claims.

Although the invention is illustrated and described herein as embodiedin one or more specific examples, it is nevertheless not intended to belimited to the details shown, since various modifications and structuralchanges may be made therein without departing from the spirit of theinvention and within the scope and range of equivalents of the claims.Accordingly, it is appropriate that the appended claims be construedbroadly and in a manner consistent with the scope of the invention, asset forth in the following claims.

1. A method for obtaining a downlink beamforming weighting vector based on channel information about an uplink channel in a wireless communications system, the method comprising: obtaining the channel information about of the uplink channel by a means selected from the group comprising training signals, pilot signals, and data signals, wherein the uplink channel comprises a set of uplink sub-channels; calculating a spatial signature of the uplink channel with the channel information; and computing a downlink beamforming weighting vector of a downlink channel with the spatial signature of the uplink channel, wherein the downlink channel comprises a set of downlink sub-channels that share few or no sub-carriers with the set of uplink sub-channels.
 2. The method of claim 1, wherein the calculating comprises: calculating one or more coefficients of a time-domain channel impulse response function of the uplink channel; and taking the Fast Fourier Transform of the time-domain channel impulse response function for a pre-determined number of sub-channels of the entire frequency band.
 3. The method of claim 2, wherein the calculating one or more coefficients of the time-domain channel impulse response function comprises: obtaining an uplink channel coefficient with a covariance method using the channel information about the uplink channel; and using the uplink channel coefficient to calculate one or more coefficients of the time-domain channel impulse response function according to a predetermined equation.
 4. The method of claim 3, wherein the predetermined equation is defined as follows: ${\begin{pmatrix} r_{g_{1}} \\ r_{g_{2}} \\ \vdots \\ r_{g_{W}} \end{pmatrix} = {\begin{pmatrix} 1 & {\exp \left( {{- {j2\pi}}\frac{g_{1}}{F}} \right)} & {\exp \left( {{- {j2\pi}}\frac{2g_{1}}{F}} \right)} & \cdots & {\exp \left( {{- {j2\pi}}\frac{\left( {M - 1} \right)g_{1}}{F}} \right)} \\ 1 & {\exp \left( {{- {j2\pi}}\frac{g_{2}}{F}} \right)} & {\exp \left( {{- {j2\pi}}\frac{2g_{2}}{F}} \right)} & \cdots & {\exp \left( {{- {j2\pi}}\frac{\left( {M - 1} \right)g_{2}}{F}} \right)} \\ \vdots & \vdots & \vdots & \; & \vdots \\ 1 & {\exp \left( {{- {j2\pi}}\frac{g_{W}}{F}} \right)} & {\exp \left( {{- {j2\pi}}\frac{g_{W}}{F}} \right)} & \cdots & {\exp \left( {{- {j2\pi}}\frac{\left( {M - 1} \right)g_{W}}{F}} \right)} \end{pmatrix}\begin{pmatrix} a_{1} \\ a_{2} \\ \vdots \\ \vdots \\ a_{M} \end{pmatrix}}},$ where r_(g) _(i) is the (equivalent) received signal on frequency g_(i), F is the size of the Fast Fourier Transform (FFT) of an OFDMA system; and a₁ is the coefficient of the i-th multi-path component of the time-domain channel impulse response function.
 5. The method of claim 4, wherein the predetermined equation is solved by a means selected from the group of matrix operations comprising inverse and pseudo-inverse matrix operations.
 6. The method of claim 4, wherein the predetermined equation is solved by a means selected from the group of estimation techniques comprising a maximum likelihood, a minimum mean squares error, and a maximum a posteriori methods.
 7. A method for obtaining a downlink beamforming weighting vector based on channel information of an uplink channel in a wireless communications system, the method comprising: obtaining the channel information about the uplink channel by a means selected from the group comprising training signals, pilot signals, and data signals, wherein the uplink channel comprises a set of uplink sub-channels; calculating a spatial signature of the uplink channel with the channel information; and computing a downlink beamforming weighting vector of a downlink channel with the spatial signature of the uplink channel, wherein the downlink channel comprises a set of downlink sub-channels that fall in neighborhoods of one or more uplink sub-channels comprising a number of sub-carries.
 8. The method of claim 7, wherein the neighborhood of one or more uplink sub-channels comprises a predetermined number of sub-carriers with a predetermined sub-channel being the center of the neighborhood.
 9. The method of claim 8, wherein the neighborhood is redefined as an. asymmetric neighborhood, if one or more sub-carriers in an uplink sub-channel overlap with one or more sub-carriers in another uplink sub-channel.
 10. The method of claim 9, wherein the asymmetric neighborhood is based on a predetermined sub-carrier of the uplink sub-channel.
 11. The method of claim 7, wherein the computing the downlink beamforming weighting vector of the downlink channel further comprising: computing the downlink beamforming weighting vector of the downlink sub-channels with the uplink sub-channel information under the condition that the sub-carriers in the downlink sub-channels fall within one or more neighborhoods of the uplink sub-channels; and constructing the downlink beamforming weighting vector of the downlink sub-channels with the channel information about the immediate neighborhoods of the uplink sub-channels under the condition that the sub-carriers in the downlink sub-channels fall outside one or more neighborhoods of the uplink sub-channels.
 12. The method of claim 11, wherein the constructing the downlink beamforming weighting vector comprises a means selected from the group consisting of interpolation and extrapolation based on the immediate neighboring uplink sub-channels.
 13. A method for obtaining a downlink beamforming weighting vector based on channel information of an uplink channel in a wireless communications system, the method comprising: obtaining the channel information about the uplink channel by a means selected from the group comprising training signals, pilot signals, and data signals, wherein the uplink channel comprises a set of uplink sub-channels; calculating a spatial signature of the uplink channel with the channel information; and computing a downlink beamforming weighting vector of a downlink channel with the spatial signature of the uplink channel, wherein the computing further comprising: computing the downlink beamforming weighting vector of the downlink sub-channels with the uplink sub-channel information under the condition that the sub-carriers in the downlink sub-channels fall within one or more neighborhoods of the uplink sub-channels; and constructing the downlink beamforming weighting vector of the downlink sub-channels with the channel information about the immediate neighborhoods of the uplink sub-channels under the condition that the sub-carriers in the downlink sub-channels fall outside one or more neighborhoods of the uplink sub-channels.
 14. The method of claim 13, wherein the neighborhood of one or more uplink sub-channels comprises a predetermined number of sub-carriers with a predetermined sub-channel being the center of the neighborhood.
 15. The method of claim 14, wherein the neighborhood is redefined as an asymmetric neighborhood, if one or more sub-carriers in an uplink sub-channel overlap with one or more sub-carriers in another uplink sub-channel.
 16. The method of claim 15, wherein the asymmetric neighborhood is based on a predetermined sub-carrier of the uplink sub-channel.
 17. The method of claim 13, wherein the constructing the downlink beamforming weighting vector comprises a means selected from the group consisting of interpolation and extrapolation. 